The present invention relates to a reading lens to be employed in an image scanner or the like, and more particularly, to a reading lens whose chromatic aberration is suppressed so as to be applicable to a color image scanner or the like.
In accordance with recent requirement of reductions in size and/or price of image scanners, fax machines or the like, it has also been required that the size and/or the price of a reading lens applicable to such an apparatus is to be reduced. An example of the reading lens is disclosed in Japan patent provisional publication No. HEI 5-119255 which teaches a triplet-type reading lens. In the publication, the costs of a triplet-type reading lens is reduced by employing two plastic lenses.
In the reading lens disclosed in the above publication, since the plastic lenses have intensive power, the reading lens is designed to compensate variation of power of the plastic lenses by assigning positive and negative powers to respective plastic lenses to counterbalance the power variation thereof due to change of temperature. However, considerable deterioration in performance due to power variation of the plastic lens may be caused as environmental conditions other than temperature, e.g., humidity changes, and such conditions should also be taken into consideration.
Further, in the above-described conventional reading lens, despite the lens having an intensive negative power is employed, longitudinal chromatic aberration is relatively large, and therefore it is not applicable to a color image scanner or the like.
Another example of a conventionally known triplet-type reading lens is configured such that a glass lens having an intensive positive power and a pair of plastic lenses respectively having relatively weak positive and negative powers. In such a reading lens, since the power of the negative lens is weak, longitudinal chromatic aberration is large, and therefore it can not be employed as a reading lens in a color image scanner or the like.